1. Field of the Invention
The present invention relates to an electronic circuit for driving an electrochromic element which utilizes fewer components; is less expensive to manufacture and provides a relatively better response time than known DC drive circuits for such electrochromic elements and more particularly to a drive circuit which generates a modulated signal, such as a pulse width modulated signal (PWM), without signal averaging which utilizes active loading or alternatively feedback of the electrochromic element to provide a reflectance response characteristic with an uncompressed linear range similar to a reflectance response characteristic for an electrochromic element with a DC drive circuit.
2. Description of the Prior Art
Various electrochromic mirror and window systems (herein after "electrochromic elements") are generally known in the art. Such systems normally include one or more electrochromic elements. For example, in automotive applications, electrochromic elements are known to be used for both the rearview mirror and one or more of the sideview mirrors. Electrochromic elements are also known to be used in window applications for sun load and privacy control.
It is known that the reflectance of an electrochromic element is a function of the voltage applied to the electrochromic element, for example, as generally described in U.S. Pat. No. 4,902,108, assigned to the same assignee as the assignee of the present invention and hereby incorporated by reference. Because of this characteristic, such electrochromic elements are known to be used to automatically control glare from external light sources in various automotive and non-automotive applications.
As discussed in copending U.S. patent application, Ser. No. 08/825,768, filed on Apr. 2, 1997, assigned to the same assignee of the present invention and hereby incorporated by reference, such electrochromic elements are known to be driven by a DC voltage source. More particularly, as discussed in detail in the aforementioned co-pending patent application, a pulse width modulated (PWM) signal is normally used as a glare control signal in order to control the reflectance level of the electrochromic element. In order to convert the PWM glare control signal to a DC signal for controlling the electrochromic element, a relatively large (i.e., 10 .mu.f) electrolytic capacitor is used to average the PWM signal and provide a DC signal for controlling the electrochromic element.
There are several disadvantages associated with the use of the electrolytic capacitor for PWM signal averaging. First of all, the electrolytic capacitor adds to the part count of the circuitry and thus, increases the complexity as well as the cost of the circuit. Secondly, the electrolytic capacitor increases the overall response time. Thus, the response time of the element is a function of the electrolytic capacitor as well as the characteristics of the electrochromic element The capacitance provided by the electrolytic capacitor thus increases the response time of the EC system. As discussed below, drive circuits for electrochromic elements which do not utilize such averaging capacitors have not heretofore been known to be used. In particular, the complex load presented by the electrochromic element has not heretofore been known to be driven by a PWM signal without signal averaging because of the compressed operating range of the reflectance characteristic of an electrochromic element when driven by such a PWM signal. More particularly, with reference to FIG. 1, the curve 1a represents an exemplary reflectance characteristic for an electrochromic element that is driven by a DC control signal is illustrated. As shown, the operating range represents that range of voltages where the reflectance characteristic is linear. As shown in the exemplary characteristic, the operating range spans from, for example, 0.4 to 1.0 volts. Such an operating range is sufficient to provide adequate control of the electrochromic element.
The curve 1b in FIG. 1 illustrates an exemplary reflectance characteristic of an electrochromic element that is driven by a PWM without signal averaging. As shown, the operating range is significantly compressed relative to the operating range illustrated by the curve 1a. As shown, the linear operating range of the exemplary reflectance characteristic of the curve 1b is compressed between 10% and 20% of the duty cycle of the PWM signal. The compressed range of the operating range illustrated in by the curve 1b is inadequate to provide adequate control of the reflectance characteristic of the electrochromic mirrors. As such, such electrochromic mirrors have utilized signal averaging capacitors forming DC drive circuits at the expense of the added complexity, cost and slower response time of the DC drive circuits for such electrochromic elements.